Magnetically induced local lattice anomalies and low-frequency fluctuations in the Mott insulator ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Fe}}_2{\mathrm{Se}}_2$

We report a study of the Mott insulator ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Fe}}_2{\mathrm{Se}}_2$ by means of ${}^{139}\mathrm{La}$ nuclear quadrupole resonance (NQR). The NQR spectra evidence a single La site in the paramagnetic phase and two inequivalent La sites, La1 and La2, in the antiferromagnetic phase. These two sites are characterized by different quadrupole couplings, indicative of distinct lattice configurations segregated in domains. The dependence of the quadrupole coupling for La2 on temperature suggests that the structural distortion is driven by the magnetic order parameter. The nuclear transverse relaxation rate $1/T_2$ evidences fluctuations in the paramagnetic phase with characteristic frequencies well below the Heisenberg exchange frequency and likely associated with nematic fluctuations.

Figure 1

(a) shows the NQR spectrum at 92 K in the frequency range between $2{\nu }_{{}_{\mathrm{Q}}}$ and $3{\nu }_{{}_{\mathrm{Q}}}$, proving the disappearance of the La2 signal for $T\ge T_{\mathrm{N}}$. NQR spectra at several temperatures below $T_{\mathrm{N}}$ are shown in (b) and (c) ($3{\nu }_{{}_{\mathrm{Q}}}$ and $2{\nu }_{{}_{\mathrm{Q}}}$, respectively). Orange and dark red lines are best-fitting Lorentzian functions for La1 and La2, respectively.

Figure 2

(a) shows the temperature dependence of the central resonance frequency of the La1 and La2 signals (solid squares and circles, respectively) at $3{\nu }_{{}_{\mathrm{Q}}}$. Open squares and circles represent the central resonance frequency of the La1 and La2 signals, respectively, at $2{\nu }_{{}_{\mathrm{Q}}}$ multiplied by a factor of 1.5. (b) reports the signal intensity $M\left(0\right)T$ for the La1 and La2 signals at $3{\nu }_{{}_{\mathrm{Q}}}$ as a function of temperature. The sharp anomaly at $\sim 60$ K is due to the crossing of La1 and La2 signals (see Fig. 1), while the peak at 110 K is an artifact associated with the maximum in the transverse relaxation rate (see Fig. 4).

Figure 3

(a) shows the spin-lattice relaxation rate for La1 and La2 (solid squares and circles, respectively) at $3{\nu }_{{}_{\mathrm{Q}}}$ as a function of temperature (log-log scale). The gray line highlights the $1/T_1\propto T^{3.4}$ power law behavior discussed in the text. (b) highlights the temperature dependence of $1/T_1$ for the La1 signal in the paramagnetic regime (linear-linear scale). The dashed horizontal line is a guide to the eye. In both panels, the dashed vertical line locates $T_{\mathrm{N}}$.

Figure 4

(a) shows ${\left(1/T_2\right)}^{\mathrm{Hahn}}$ for La1 and La2 (solid squares and circles, respectively) at $3{\nu }_{{}_{\mathrm{Q}}}$ as a function of temperature. The gray line is a best fit according to an activated Arrhenius law. The open triangles show the temperature dependence of ${\left(1/T_2\right)}^{\mathrm{CPMG}}$ at fixed pulse separation (${\tau }_{{}_{\mathrm{CPMG}}}=12$ $\mu \mathrm{s}$). (b) shows the effect of increasing the pulse separation on ${\left(1/T_2\right)}^{\mathrm{CPMG}}$. The gray line is a linear best fit to the data.